mysql的order by rand方法实际使用

发布时间：2023-09-01 15:26:19 所属栏目：MySql教程来源：

导读：从一次查询中随机返回一条数据，一般使用mysql的order by rand() 方法来实现

例如: 从20万用户中随机抽取1个用户

mysql> select * from user order by rand() limit 1;

+-------+------------+---------

从一次查询中随机返回一条数据，一般使用mysql的order by rand() 方法来实现

例如: 从20万用户中随机抽取1个用户

mysql> select * from user order by rand() limit 1;

+-------+------------+----------------------------------+----------+--------------+-----------+| id    | phone      | password                         | salt     | country_code | ip        |

+-------+------------+----------------------------------+----------+--------------+-----------+| 15160 | 6549721306 | e4f302120c006880a247b652ad0e42f2 | 40343586 | 86           | 127.0.0.1 |

+-------+------------+----------------------------------+----------+--------------+-----------+1 row in set (0.25 sec)mysql> explain select * from user order by rand() limit 1;

+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows   | Extra                           |

+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+| 1 | SIMPLE      | user | ALL | NULL          | NULL | NULL    | NULL | 200303 | Using temporary; Using filesort |

+----+-------------+-------+------+---------------+------+---------+------+--------+---------------------------------+1 row in set (0.00 sec)

根据分析结果，运行需要0.25秒，order by rand() 需要使用临时表（Using temporary），需要使用文件排序（Using filesort），效率低下。

改进方法

mysql的order by rand方法实际运用

1.首先获取查询的总记录条数total

2.在总记录条数中随机偏移N条（N=0~total-1)

3.使用limit N,1 获取记录

代码如下：

<?php// 获取总记录数$sqlstr = 'select count(*) as recount from user';$query = mysql_query($sqlstr) or die(mysql_error());$stat = mysql_fetch_assoc($query);$total = $stat['recount'];// 随机偏移$offset = mt_rand(0, $total-1);// 偏移后查询$sqlstr = 'select * from user limit '.$offset.',1';$query = mysql_query($sqlstr) or die(mysql_error());$result = mysql_fetch_assoc($query);

print_r($result);?>

分析：

mysql> select * from user limit 23541,1;

+-------+------------+----------------------------------+----------+--------------+-----------+| id    | phone      | password                         | salt     | country_code | ip        |

+-------+------------+----------------------------------+----------+--------------+-----------+| 23542 | 3740507464 | c8bc1890de179538d8a49cc211859a46 | 93863419 | 86           | 127.0.0.1 |

+-------+------------+----------------------------------+----------+--------------+-----------+1 row in set (0.01 sec)mysql> explain select * from user limit 23541,1;

+----+-------------+-------+------+---------------+------+---------+------+--------+-------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows   | Extra |

+----+-------------+-------+------+---------------+------+---------+------+--------+-------+| 1 | SIMPLE      | user | ALL | NULL          | NULL | NULL    | NULL | 200303 | NULL |

+----+-------------+-------+------+---------------+------+---------+------+--------+-------+1 row in set (0.00 sec)

（编辑：聊城站长网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!